Over the last decade, there has been an increasing interest in discovering alternative sources of fuels and chemicals from resources other than petroleum. Development of non-petroleum-based fuels may provide economic and environmental benefits, while also increasing national security by decreasing reliance on non-domestic energy sources. Biomass, such as plants and animal fats, represent a major alternative source of hydrocarbons that can be converted into fuels. Liquid fuels derived from biomass are rapidly entering the market, driven by both need for increased national energy independence and rapid fluctuations in the cost of petroleum products. In 2007, the Energy Independence and Security Act was passed in the United States, which requires increasing quantities of bio-derived fuels to be produced over time. Similarly, the European Union directive 2003/30/EC promotes the use of biofuels or other renewable fuels. The directive has set a minimum percentage of biofuels to replace diesel or gasoline for transport purposes, such that by 2011 a 5.75% minimum proportion of Biofuels will be required in all gasoline and diesel fuels sold. Thus, it is essential to develop more efficient processes to convert bio-derived compounds into fuels that can fulfill these government mandates, as well as future global energy needs.
The carbohydrates found in plants and animals can be used to produce fuel range hydrocarbons. However, many carbohydrates (e.g., starch) are undesirable as feed stocks for creating biomass-derived fuels due to the costs associated with converting them to a usable form. The chemical structure of some carbohydrates makes them difficult to convert, and conversion processes may produce low yields of desirable products. Carbohydrates that are difficult to convert include compounds with low effective hydrogen to carbon ratios, including carbohydrates such as starches and sugars, as well as other oxygenates with low effective hydrogen ratios such as carboxylic acids and anhydrides, light glycols, glycerin and other polyols and short chain aldehydes. Therefore, development of an efficient and inexpensive process for converting one or more of these difficult-to-convert biomass feedstocks into a form suitable for use as an oxygenated fuel additive could be a significant contribution to the art and to the economy.
The first step in processing biomass is to cleave larger structures down to their component subunits. Some processes, such as acid hydrolysis, can release the smaller pentose and hexose subunits from larger structures such as cellulose and starch. Because these sugars are inherently five to six carbons in length, a complete deoxygenation process yields saturated hydrocarbons having boiling points in the gasoline “boiling-range” (i.e., about 27° C. to 190° C.). Unfortunately, limited options are available for the upgrading/conversion of saturated five and six carbon hydrocarbons, and no options currently exist for converting these materials into liquid hydrocarbon fuels that boil in the diesel range in conventional refining units. Alternatively, partial deoxygenation of these sugars leaves some oxygen in the starting material, providing a variety of opportunities for upgrading due to the numerous chemical reactions that oxygenates may undergo.
Several partially-deoxygenated intermediates are available in these biomass feeds that may be converted to products useful as oxygenated fuel additives. One class of intermediates includes five and six carbon diols derived from pentose and hexose sugars. These diols are not suitable for direct blending into fuels as they have a low cetane number, as well as low miscibility in hydrocarbon fuels.
Condensation reactions are one way to assemble oxygenates into larger compounds. Condensation allows the conversion of small bio-derived feedstocks into a larger size that is better-suited for use as a fuel additive. Several groups have reported the conversion of certain biomass-derived feedstocks via condensation reactions. A paper by Karinen, et. al. pertains to the etherification of glycerol and isobutene, while papers by Frustieri, et. al. and Keplacova, et. al, both report methods for catalytic etherification of glycerol by tert-butyl alcohol. US2010/0094062 describes a process for the etherification of glycerol with an alkene or alkyne, followed by nitration of a remaining hydroxyl group. A portion of the process claimed in US2008/0300435 pertains to the dimerization/condensation of monofunctional alcohols such as pentanol or isopropyl alcohol, while US2008/0302001 pertains to methods for producing biofuels that include several types of condensation reactions, including the Guerbet alcohol condensation, but not an acidic condensation of two hydroxyls to form an ether. To date, no methods have demonstrated an efficient process for the acid condensation of a feedstock comprising diols containing five and six carbons to produce an oxygenated fuel additive.